Along with the advancement of IT and the spreading of the Internet, the amount of data handled by computer systems of companies and the like is increasing continuously, and storage subsystems storing such data are required to have high performance.
In the past, various techniques have been introduced to enhance the data access performance. One of such techniques is DMA (Direct Memory Access). A DMA engine is provided to a host computer and/or a storage subsystem, and data transfer processing is made to be performed by the DMA engine instead of the CPU in the host computer or the storage subsystem. Thereby, the overhead of CPU in the host computer or the storage subsystem can be reduced, and as a result, the performance of the system can be improved. For example, according to Patent Literature 1, a host writes a command to a storage when transferring (read or write) data between the host and the storage. A technique is disclosed where, based on the written command, the storage makes the DMA engine to transfer the access target data stored in the memory of the host to a storage area (memory and the like) in the storage.
Another conventional technique is a cache technique. By storing write data from the host computer or data frequently accessed from the host computer to a cache memory composed of DRAMs and the like, the access performance can be improved compared to when access is performed to a storage device having a low access performance, such as HDDs or SSDs.